Amount of clockwise rotation of Southwest Japan — fan shape opening of the southwestern part of the Japan Sea

Paleomagnetic measurements have been carried out on welded tuffs ranging in age between 58 Ma and 112 Ma from Yamaguchi and Go¯river areas in the central part of Southwest Japan. The new data, together with those of younger igneous rocks published previously, define the change of paleomagnetic field direction during the late Mesozoic/ Cenozoic period for Southwest Japan. The paleomagnetic direction from this area has pointed 56 ± 3° clockwise from the expected field direction estimated from APWP (apparent polar wandering path) of the whole of Eurasia during the period between 100 and 20 Ma. In comparison with the expected one from the eastern margin of Eurasia (Korea, China, Siberia), the Cretaceous field direction of Southwest Japan shows the clockwise deflection by 44–49°. These results establish that while the eastern margin of Eurasia, including Southwest Japan, was rotated more or less with respect to the main part of Eurasia during last 100 Ma, Southwest Japan was rotated clockwise through more than 40° with respect to the eastern margin of Eurasia since 20 Ma. The large amount of rotation for Southwest Japan implies that it is rotated by an opening of the southwestern part of the Japan Sea, which widens northeastward (fan shape opening). The tectonic feature of Southwest Japan and the Japan Sea is analogous to that of Corso-Sardinia and the Ligurian Sea in the Mediterranean, indicating that the fan shape opening is a specific feature of the rifting of the continental sliver at the continental rim.     

Miocene intra-arc bending at an arc-arc collision zone, central Japan

Abstract Recent paleomagnetic studies are reviewed in an effort to clarify the relationship between the intra-arc deformation of central Japan and the collision tectonics of the Izu-Bonin Arc. The cusp structure of the pre-Neogene terranes of central Japan, called the Kanto Syntaxis, suggests a collisional origin with the Izu-Bonin Arc. The paleomagnetic results and newly obtained radiometric ages of the Kanto Mountains revealed the Miocene rotational history of the east wing of the Kanto Syntaxis. More than 90° clockwise rotation of the Kanto Mountains took place after deposition of the Miocene Chichibu Basin (planktonic foraminiferal zone of N.8: 16.6–15.2 Ma). After synthesizing the paleomagnetic data of the Japanese Islands and collision tectonics of central Japan, it appears that approximately a half rotation (40–50°) probably occurred at ca 15 Ma in association with the rapid rotation of Southwest Japan. The remainder (50-40°) continued until 6 Ma, resulting in the sharp bent structure of the pre-Neogene accretionary complexes (Kanto Syntaxis). The latter rotation seems to have been caused by the collision of the Izu-Bonin Arc on the northwestward migrating Philippine Sea Plate.     

K–Ar ages of the basaltic rocks from Far East Russia: Constraints on the tectono-magmatism associated with the Japan Sea opening

K–Ar ages of the Cenozoic basaltic rocks from the Far East region of Russia (comprising Sikhote-Alin and Sakhalin) are determined to obtain constraints on the tectono-magmatic evolution of the Eurasian margin by comparison with the Japanese Islands, Northeast China, and the formation of the back-arc basin. In the early Tertiary stage (54–26 Ma), the northwestward subduction of the Pacific Plate produced the active continental margin volcanism of Sikhote-Alin and Sakhalin, whereas the rift-type volcanism of Northeast China, inland part of the continent began to develop under a northeast–southwest-trending deep fault system. In the early Neogene (24–17 Ma), a large number of subduction-related volcanic rocks were erupted in connection with the Japan Sea opening. After an inactive interval of the volcanism ∼ 20–13 Ma ago, the late Neogene (12–5 Ma) volcanism of Sikhote-Alin and Sakhalin became distinct from those of the preceding stages and indicated within-plate geochemical features similar to those of Northeast China, in contrast to the Japan Arc which produces island arc volcanism. During the Japan Sea opening, the northeastern Eurasian margin detached and became a continental island arc system, and an integral part of continental eastern Asia comprising Sikhote-Alin, Sakhalin and Northeast China, and the Japan Arc with a back-arc basin. The convergence between the Eurasian Plate, the Pacific Plate and the Indian Plate may have contributed to the Cenozoic tectono-magmatism of the northeastern Eurasian continent.     

Paleomagnetic evidence for the clockwise rotation of Southwest Japan

Over 500 oriented samples of felsic rocks of Cretaceous to Middle Miocene age were collected along the Go¯River in the central part of Southwest Japan, in an attempt to detect the process of tectonic rotation of Southwest Japan from the paleomagnetic view point. Thermal demagnetization was successful in isolating characteristic directions from the remanent magnetization of samples. Reliability of the paleomagnetic direction is ascertained through the agreement of directions from different kinds of rocks as well as the presence of both normal and reversed polarities. The paleomagnetic results establish that Southwest Japan began to rotate clockwise through58 ± 14° later than 28 Ma and ceased its motion by about 12 Ma. Southwest Japan has undergone no detectable north-south translation since 28 Ma. These results imply that southwest Japan was rotated about the pivot around 34°N, 129°E between 28 Ma and 12 Ma in association with the opening of the Japan Sea.     

Paleoposition of Southwest Japan at 16 Ma: Implication from paleomagnetism of the Miocene Ichishi Group

New paleomagnetic data from shallow-marine sediments of the Ichishi Group suggest a clockwise tectonic rotation of Southwest Japan in the Middle Miocene. Samples have been collected from mud or tuff layers at 17 sites. Stability of remanent magnetization has been examined by using alternating field and thermal demagnetization. The polarity sequence, composed of four normal and seven reversed polarity sites, is correlated to Polarity Epoch 16 (15.2–17.6 Ma), based on micropaleontological assignment of the upper Ichishi Group to Blow's Zone N8. The mean paleomagnetic direction of the 11 sites shows an anomalous declination toward the northeast. This result suggests that Southwest Japan was subjected to a clockwise rotation through 45° since 16 Ma. The clockwise rotation can be explained by the drift of Southwest Japan associated with the spreading of the Japan Sea during the Middle Miocene.     

Collisional bending of the western Paleo‐Kuril Arc deduced from paleomagnetic analysis and U–Pb age determination

The Paleo‐Kuril Arc in the eastern Hokkaido region of Japan, the westernmost part of the Kuril Arc in the northwestern Pacific region, shows a tectonic bent structure. This has been interpreted, using paleomagnetic data, to be the result of block rotations in the Paleo‐Kuril Arc. To understand the timing and origin of this tectonic bent structure in the Paleo‐Kuril arc‐trench system, paleomagnetic surveys and U–Pb radiometric dating were conducted in the Paleogene Urahoro Group, which is distributed in the Shiranuka‐hill region, eastern Hokkaido. The U–Pb radiometric dating indicated that the Urahoro Group was deposited at approximately 39 Ma. Paleomagnetic analysis of the Urahoro Group suggested that the Shiranuka‐hill region experienced a 28° clockwise rotation with respect to East Asia. The degree of clockwise rotation implied from the Urahoro Group is smaller than that of the underlying Lower Eocene Nemuro Group (62°) but larger than that of the overlying Onbetsu Group (?9°). It is thus suggested that the Shiranuka‐hill region experienced a clockwise rotation of approximately 34° between the deposition of the Nemuro and Urahoro Groups (50–39 Ma), and a 38° clockwise rotation between the deposition of the Urahoro and Onbetsu Groups (39–34 Ma). The origin of the curved tectonic belt of the Paleo‐Kuril Arc was previously explained by the opening of the Kuril Basin after 34 Ma. The age constraint for the rotational motion of the Shiranuka‐hill region in this study contradicts this hypothesis. Consequently, it is suggested that the process of arc–arc collision induced the bent structure of the western Paleo‐Kuril Arc.     

Timing of rotational motion of Southwest Japan inferred from paleomagnetism

Paleomagnetic field directions for the period younger than 35 Ma are obtained from igneous rocks distributed in the San'in district, Inner Zone of Southwest Japan. The remanent magnetization of samples between 30 and 35 Ma old are fairly well grouped with a mean direction of D = 65.9°, I = 48.6°, and ₉₅ = 6.5°. This result establishes that Southwest Japan rotated clockwise 56 ± 12° during the past 30 m.y. A declination value of about 60° is observed in the rocks of 28 Ma ( D = 52.2°, I = 33.5°) and 21 Ma (D = 69.9°, I = 49.5°). Comparing this with results from dacitic rocks with an age of 15 Ma in other areas of Southwest Japan suggests that rotational motion did not occur possibly until 15 Ma. These results require that the rotation of Southwest Japan occurred just after the Shikoku Basin had been created.     

Volcanic history and tectonics of the Southwest Japan Arc

Abstract Remarkable changes in volcanism and tectonism have occurred in a synchronous manner since 1.5–2 Ma at the junction of the Southwest Japan Arc and the Ryukyu Arc. Although extensive volcanism occurred in Kyushu before 2 Ma, the subduction-related volcanism started at ca 1.5 Ma, forming a NE–SW trend volcanic front, preceded by significant changes in whole-rock chemistry and mode of eruptions at ca 2 Ma. The Median Tectonic Line has intensified dextral motion since 2 Ma, with a northward shift of its active trace of as much as 10 km, accompanied by the formation of rhomboidal basins in Central Kyushu. Crustal rotation and incipient rifting has also occurred in South Kyushu and the northern Okinawa Trough over the past 2 million years. We emphasize that the commencement age of these events coincides with that of the transition to the westward convergence of the Philippine Sea plate, which we interpret as a primary cause of these synchronous episodes. We assume that the shift in subduction direction led to an increase of fluid component contamination from subducted oceanic slab, which then produced island-arc type volcanism along the volcanic front. Accelerated trench retreat along the Ryukyu Trench may have caused rifting and crustal rotation in the northern Ryukyu Arc.     

Magnetostratigraphic dating of Early Miocene deep-sea fossils from the Morozaki Group in central Japan

Early Miocene sediments of the Morozaki Group in central Japan contain deep-sea fossils that have been dated using biostratigraphic and radiometric data. In this study, we utilize magnetostratigraphy to provide a more precise age for mudstones from just below the layer containing the fossils. Rock magnetic experiments suggest that both magnetic iron sulfide and Ti-poor titanomagnetite carry the remanent magnetization of the mudstones. Two different stratigraphic sites have normal polarity directions with a northeastern declination, which can be correlated with Chronozone C5Dn. Given their magnetostratigraphic position near the C5Dn/C5Dr chronozone boundary (17.466 Ma) and a high sedimentation rate, the estimated age for both the sites and the deep-sea fossils is ~17.4 Ma. The northeasterly-directed site-mean directions suggest clockwise tectonic rotation, most likely due to the Early Miocene clockwise rotation of Southwest Japan associated with the back-arc opening of the Japan Sea. The deep-sea fossils, dated at ~17.4 Ma, represent organisms deposited within a submarine structural depression formed by crustal extension during the back-arc opening stage.     

Rapid tectonics of the Late Miocene Boso accretionary prism related to the Izu–Bonin arc collision

Abstract   The structure, paleomagnetism and biostratigraphy of the Nishizaki and Kagamigaura formations on the southern Boso Peninsula, central Japan, were investigated to determine the chronographic constraints on the accretion, post-Late Miocene rotation and regional tectonics in the Izu–Bonin island arc collision zone. The geological structures on the southern Boso Peninsula are characterized by an east–west trending and south-verging fold and thrust belt that curves toward the northwest–southeast in the northwest extent of the Nishizaki Formation. Two stages of tectonic rotation were revealed by paleomagnetic and structural studies. The first is believed to have occurred after the accretion of the Nishizaki Formation and before the deposition of the Kagamigaura Formation, while the second is confidently correlated with the 1 Ma Izu block collision. The northwest extent of the Nishizaki Formation was rotated clockwise by approximately 65–80°, whereas the rotation was only 25–30° in the east, and 11–13° in the overlying Kagamigaura Formation. Radiolarian biostratigraphy suggests a depositional age of 9.9–6.8 Ma (Upper Miocene period) for the Nishizaki Formation and 4.19-3.75 Ma (Pliocene period) for the lower Kagamigaura Formation. These results indicate that the age of accretion and first-stage rotation of the Nishizaki Formation can be constrained to the interval of 6.80–3.75 Ma. This structure most likely represents the northward bending caused by collisions of the Tanzawa and Izu blocks with the Honshu island arc, and suggests rapid processes of accretion, collision, uplift and the formation of new sedimentary basins within a relatively short period of time (2.61–3.05 my).     

Spatial distribution of the apatite fission‐track ages in the Toki granite,central Japan: Exhumation rate of a Cretaceous pluton emplaced in the East Asian continental margin

The Cretaceous Toki granitic pluton of the Tono district, central Japan was emplaced in the East Asian continental margin at about 70 Ma. The Toki granite has apatite fission‐track (AFT) ages ranging from 52.1 ±2.8 Ma to 37.1 ±3.6 Ma (number of measurements, n = 33); this indicates the three‐dimensional thermal evolution during the pluton's low‐temperature history (temperature in the AFT partial annealing zone: 60–120 °C). The majority of the Toki granite has a spatial distribution of older ages in the shallower parts and younger ages in the deeper parts, representing that the shallower regions arrived (were exhumed) at the AFT closure depth earlier than the deeper regions. Such a cooling pattern was predominantly constrained by the exhumation of the Toki granitic pluton and was related to the regional denudation of the Tono district. The age–elevation relationships (AERs) of the Toki granite indicate a fast exhumation rate of about 0.16 ±0.04 mm/year between 50 Ma and 40 Ma. The AFT inverse calculation using HeFTy program gives time‐temperature paths (t–T paths), suggesting that the pluton experienced continuous slow cooling without massive reheating since about 40 Ma until the present day. A combination of the AERs and AFT inverse calculations represents the following exhumation history of the Toki granite: (i) the fast exhumation at a rate of 0.16 ±0.04 mm/year between 50 Ma and 40 Ma; (ii) slow exhumation at less than 0.16 ±0.04 mm/year after 40 Ma; and (iii) exposure at the surface prior to 30–20 Ma. The Tono district, which contains the Toki granite, underwent slow denudation at a rate of less than 0.16 ±0.04 mm/year within the East Asian continental margin before the Japan Sea opening at 25–15 Ma and then within the Southwest Japan Arc after the Japan Sea opening, which is in good agreement with representative denudation rates obtained in low‐relief hill and plain fields.     

Paleogeographic changes in central Honshu, Japan, during the late Cenozoic in relation to the collision of the Izu-Ogasawara Arc with the Honshu Arc

Abstract The northern tip of the Izu-Ogasawara Arc on the Philippine Sea plate collided with the central part of the Honshu Arc in the early Quaternary. The collision history is recorded in late Cenozoic strata that are distributed widely in central Japan. To reconstruct paleotopography during the collision process, paleogeographic maps of central Japan were drawn at six time slices during the late Cenozoic. These maps were made from paleodepth data that were inferred from benthic foraminiferal fossil assemblages. Sedimentological information was also added to the maps. The paleogeographic maps show several distinctive geological features. The paleodepth of the area between the Izu-Ogasawara Arc and the Honshu Arc changed quickly from deep-sea to shallow marine during the Quaternary by means of rapid deposition of large amounts of coarse-grained detritus. The conglomerate was first deposited in a trough as deep-sea fan deposits, and filled the trough until an alluvial fan was formed. Forearc basins of the Honshu Arc facing the collision area subsided from 3 to 1 Ma. Vertical movement of the basin was inferred from a strata thickness/paleodepth correlation graph. It is thought that the tectonic inversion seen in sedimentary basins in the Kanto and Tokai regions might be closely related to the change of motion of the Philippine Sea plate. However, a basin of the Ashigara area sunk continuously without interruption until 0.7 Ma. The collision event affected strongly distribution of deep-sea benthic foraminifera. Paleogeographic maps show that a deep trough appeared in the Ashigara area between 3 and 1.5 Ma. This trough may have served as a passage for the migration of deep-sea benthic foraminifera between the Pacific and the Philippine Sea. Cold water benthic foraminiferal species occur west of the Izu-Ogasawara ridge deposited in strata during the existence of the passage.     

Termination of intra‐arc rifting at ca 16 Ma in the Southwest Japan arc: The tectonostratigraphy of the Hokutan Group

Miocene intra‐arc rifting associated with the opening of the Japan Sea formed grabens in several areas in Southwest (SW) Japan, but the extensional tectonics of the arc are still not well understood. In this study, we first document the tectonostratigraphy of the Hokutan Group in the northwestern part of the Kinki district, and demonstrate the termination of extensional tectonics at ca 16.5 Ma, as inferred from grabens in the lower part of the group being unconformably overlain by sediments of the upper part. Second, we review early Miocene grabens in SW Japan to suggest that intra‐arc rifting was abandoned at ca 16 Ma, essentially simultaneously with the end of rotation of the SW Japan arc as evidenced by paleomagnetic studies. The lesser numbers of grabens and reduced thicknesses of graben fills suggest that extensional deformation of the SW Japan arc was significantly weaker than that of the Northeast (NE) Japan arc, which was broken into blocks, indicating various degrees of paleomagnetic rotation within NE Japan. The weak deformation has allowed paleomagnetic studies to infer the coherent rotation of the SW Japan arc.     

Collision orogeny at arc-arc junctions in the Japanese Islands

Abstract In the Japanese Islands, collision tectonics are operating at arc-arc junctions in three regions: Hokkaido, Central Japan and Kyushu. Hokkaido is situated at the junction of the Kuril and Northeast Japan Arcs. The Kuril fore arc sliver collides with the Northeast Japan Arc, and the lower crust of the Kuril Arc thrusts upon the fore arc of the Northeast Japan Arc in Hokkaido. Outcrops of the lower crust are observed in the Hidaka Mountains in the fore arc of the junction area. Central Japan is in the juncture area among the Northeast Japan, Izu-Bonin, and Southwest Japan Arcs. The Izu-Bonin arc is colliding against the Honshu mainland, which has been bent by the collision. Kyushu is a juvenile collision area between the Southwest Japan and Ryukyu Arcs. The fore arc of the Southwest Japan Arc is starting to underthrust beneath the Kyushu islands along the Bungo Strait, where shallow seismicity within the crust is active in terms of the collision. Collision tectonics are observed at most of the arc-arc junctions in the circum-Pacific orogenic belts and may be an important process contributing to the relatively rapid growth of new continental crust in subduction zones.     

南海新生代海底扩张的构造演化模式:来自高分辨率地球物理数据的新认识

根据高分辨率重、磁测网数据的分析,结合多波束海底地貌的构造解释,南海海盆新生代经历了两期不同动力特征的海底扩张,25 Ma的沉积-构造事件是其重要分界.早期扩张从约33.5 Ma开始至25 Ma停止,在东部海盆南、北两侧和西北海盆形成了具有近E-W向或NEE向磁条带的老洋壳,是近NNW-SSE向扩张的产物;晚期扩张从2...     

Paleomagnetic constraints on the late Cretaceous and Cenozoic tectonics of southeastern Asia

Many features of the Cenozoic tectonic history of central and southeastern Asia can be understood as direct consequences of the thrust and penetration of India into Asia. Recent indentation experiments with plasticine (Tapponnier et al. [7]) have extended this idea and have led to the prediction of a pattern of large rotations and displacements of continental blocks that can be tested by paleomagnetism. The available Cretaceous and Cenozoic paleomagnetic data from this part of the world have been reviewed and a new APWP for Eurasia has been constructed for reference. The negligible rotation of South China and large clockwise rotation of Indochina are consistent with the model, i.e., with an history of large-scale left-lateral strike-slip motion along the Altyn Tagh and Red River faults. Data from Malaya and Borneo can be reconciled with the model, although in a less straightforward fashion. The large counter clockwise rotation of South Tibet implies that it rotated in sympathy with India during the collision and suggests that future indentation experiments should include this feature. Finally a middle Cretaceous reconstruction of the south margin of Asia is proposed. One interesting result is the restored continuity of geological features in Tibet and Indochina, with active subduction of oceanic (Indian plate) crust taking place to the south at subtropical latitudes.     

Zircon U–Pb geochronology of “Sashu mylonite”, eastern extension of Higo Plutono‐metamorphic Complex,Southwest Japan: Implication for regional tectonic evolution

Geochronological and geochemical studies reveal the possible origin of the restricted body of mylonite rocks occurring at the eastern edge of Kyushu Island, Japan, just in contact with the Sashu Fault, a part of the Paleo‐Median Tectonic Line (Paleo‐MTL). The LA‐ICP‐MS zircon U–Pb dating of the quartz diorite mylonite in this mylonitic body indicates a crystallization age of 114.0 ±1.7 Ma. Moreover, the two tonalite samples appear as thin layers within the Permian fine‐grained mafic mylonite; a part of the same body yields the age of 113.7 ±2.3 Ma and 116.9 ±1.3 Ma, with extremely low Th/U ratio. These quartz diorite mylonite and tonalite are consistent with the late Early Cretaceous magmatism and coeval metamorphism similar to those in the Higo Plutono‐metamorphic Complex in western Kyushu, Japan. This newly characterized complex occurs just south of the Cretaceous Sambagawa metamorphic rocks. The newly characterized mylonitic rocks are lying structurally above the Sambagawa Metamorphic Complex and are distributed along the Paleo‐MTL. The extension of the Higo Plutonometamorphic Complex, as well as the structural relationship between this complex and the Sambagawa Metamorphic Complex, is still controversial but holds a key to reconstruct the tectonic evolution of Southwest Japan during the Late Mesozoic to Early Cenozoic period. Hence, this article provides new insight into the reconstruction of the evolution history of East Asia as an active convergent margin.     

Mesozoic evolution of West Antarctica and the Weddell Sea Basin: new paleomagnetic constraints

Paleomagnetic data from the Antarctic Peninsula and our recent results from the Ellsworth-Whitmore Mountains block suggest that since the Middle Jurassic these two West Antarctic blocks have undergone little relative movement and together have rotated relative to the East Antarctic craton. New data from Lower Cretaceous rocks from the Thurston Island region of West Antarctica suggest that on the basis of paleomagnetic constraints, the Antarctic Peninsula, Ellsworth-Whitmore Mountains and Thurston Island blocks define a single entity which we call Weddellia; some motion between these blocks is possible within the limits of the paleomagnetic data.Between the Middle Jurassic and Early Cretaceous, Weddellia remained attached to West Gondwanaland while East Antarctica moved southward (dextrally) relative to Weddellia. From the Early Cretaceous to mid-Cretaceous, Weddellia rotated clockwise 30° and moved sinistrally approximately 2500 km relative to East Antarctica, to its present-day position. We suggest the Early to mid-Cretaceous to be the time of the main if not initial opening of the Weddell Sea.     

On the possibility of a widespread remagnetization of pre-Oligocene rocks from Northeast Japan and the Miocene rotational opening of the Japan Sea

We present new paleomagnetic results from the well dated Miyako Cretaceous sediments (100–110 Ma) from Northeast Japan. These results, combined with those of Tosha [1], yield an in-situ characteristic directionD = 321°,I = 54.5° (α₉₅ = 4.5°),N = 14 sites; reduced to a reference point at 40°N, 142°E). This direction is found to coincide with that of most older plutonic and sedimentary rocks of Devonian to lower Cretaceous age. It is also identical with the westerly pre-folding direction which is preserved in many Oligocene (20–40 Ma) formations from Northeast Japan [1,2]. In contrast, all recent formations (0–17 Ma) have been magnetized in the direction of the present axial dipole field. Only the Oligocene and Miocene results appear to be primary, or at least pre-folding. The Miyako sulfide-bearing sediments and lower Cretaceous (110–125 Ma) magnetite-bearing granites could either still bear a primary magnetization or be completely remagnetized by a low temperature chemical event. Evidence for such events is now found in many places, and as close as South Korea. Available data constrain the Oligo-Miocene history of Northeast Japan and indicate at least20/30° counterclockwise rotation with respect to mainland Asia during the opening of the Sea of Japan. On the other hand, the pre-40 Ma history of Northeast Japan is not well constrained and three models are proposed which are compatible with various interpretations of the data. None of them can presently document pro-Oligocene motion of Northeast Japan with respect to Asia. The most “economical” model implies widespread remagnetization. We conclude that, because of the scarcity of well tested primary magnetization directions, the classical bending of the Japanese Islands rests on weaker grounds than generally realized and that no pre-40 Ma apparent polar wander path of the Japanese Islands can safely be proposed.     

Paleomagnetism in an accretionary margin Luzon, Northern Philippines

Paleomagnetic results from Northern and Central Luzon reveal clockwise rotated declinations for Late Miocene rocks. This is interpreted as a record of tectonic rotation when Luzon was a part of the Philippine Sea Plate, prior to the development of the East Luzon trench. To the south of Luzon a left lateral shear zone is required to separate it from regions which do not show Late Miocene clockwise rotations.Paleomagnetic data from older rocks do not give such clear regional patterns, as do the Late Miocene sites. However, both the Cretaceous Angat and the Eocene Zambales ophiolites appear to have originated at equatorial latitudes.